Supporting ECDSA with LibreSSL

[omasanori]

Background

While LibreSSL 2.7+ features partial API compatibility with OpenSSL 1.0.2 and 1.1.0, there are still missing symbols to enable ECDSA support with it. We will be able to support ECDSA with LibreSSL when missing APIs are provided, but for now, I would like to share the list below as a reminder.

Status

LibreSSL 2.9.1 provides mandatory APIs AFAIK.

How to test

Please fetch #25 and comment on the result back to the PR.

[omasanori]

Good news: While they had missed documentation until recently so I overlooked, the EC APIs have been provided since LibreSSL 2.9. Moreover, LibreSSL 3.0 will provide RSA_meth_set_sign. Thus, only RSA_PKCS1_OpenSSL is missing and we can complement the remaining one because that is just a new name of an old function.

[omasanori]

After refactoring, newer RSA APIs are no longer required to support ECDSA. (well, it is obvious, isn't it?) Refer #25 for details.